Don't know about grandfathered-in back in your area, but there is no such thing here. When an area is declared a no-discharge zone, it is no discharge for people with boats built in 1898 and boats built in 2015. No discharge is no discharge. If there were exceptions, there wouldn't be much point in the rule.
A 15 gallon holding tank is very minimal in my opinon, particularly for a live-aboard unless you are planning to pump out your tank almost every day. Obviously if you're planning on using the shore-side facilities in your home harbor most of the time this will make a major difference. But there will still be those times when getting up in the middle of the night to get dressed enough to trudge up the dock(s) to the shoreside restroom is not very appealing. Or if it's raining hard. Much easier to use a toilet on board, right?
We have two holding tanks on our boat, one for each toilet. The forward toilet goes to an approximately 40 gallon tank in the bilge between the engines. The aft toilet goes to a 25 gallon tank in the lazarette. Unfortunately, the aft head is the one that gets used the most. A 25 gallon tank given typical use by the two of us can go as much as four days before getting filled up close to the top. If we had more modern toilets, Vacuflush for example, our toilet water usage would be less.
When we have guests, really the only time the forward head gets used, the 40 gallon tank is good for a week or more, depending on toilet use.
At least, when we are boating on our own which is 95% of the time, we have the forward head and its 40-gallon tank to fall back on if the aft tank gets filled up and we've not had an opportunity to empty it.
If all we had on the boat was a 15 gallon tank, it would render our boat almost unusable for anything other than a weekend cruise.
